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Published online by Cambridge University Press: 26 February 2010
Let V = V2n be the Segre product variety
of two n–dimensional complex projective spaces. Then any Cremona transformation of Sn into
(regarded as an irreducible algebraic system of ∞n ordered pairs of points) is represented on V by an irreducible n-dimensional subvariety H which satisfies (on V) the algebraic equivalence

where Si, j is a subvariety
of V and m1, …, mn–1 are positive integers. We call m1, …, mn–1 the characters of T noting that, numerically,
